1. Field of the Invention
The present invention relates to an electromagnetic transponder, that is, a transceiver (most often, mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read and/or write terminal. Generally, transponders extract the power supply required by the electronic circuits included therein from a high frequency field radiated by an antenna of the read/write terminal. There exist read-only transponders, that is, transponders adapted to operating with a terminal which only reads transponder data, and read/write transponders, which contain data that can be modified by the terminal.
2. Discussion of the Related Art
Systems using electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna, on the transponder side and on the read/write terminal side. These circuits are capable of being coupled by close magnetic field when the transponder enters the field of the read/write terminal.
FIG. 1 very schematically shows a conventional example of a data exchange system between a read/write terminal 1 and a transponder 10 of the type to which the present invention applies.
Generally, terminal 1 is essentially formed of a series oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor Rb, between an output terminal 2 of an amplifier or antenna coupler (not shown) and a reference terminal 3 (generally the ground). The antenna coupler belongs to a circuit 4 for controlling the oscillating circuit and for exploiting the received data including, among others, a modulator-demodulator and a microprocessor for processing the control signals and the data. Circuit 4 of the terminal generally communicates with different input/output circuits (keyboard, screen, means of exchange with a server, etc.) and/or processing circuits, not shown. The circuits of the read/write terminal generally draw the power required by their operation from a supply circuit (not shown) connected, for example, to the electric supply system.
A transponder 10, capable of cooperating with a terminal 1, essentially includes a parallel oscillating circuit formed of an inductance L2, in parallel with a capacitor C2 between two A.C. input terminals 11, 12 of a control and processing circuit 13. Terminals 11 and 12 are, in practice, connected to the input of a rectifying means (not shown in FIG. 1) the outputs of which form D.C. supply terminals of the circuits internal to the transponder. These circuits generally include, essentially, a microprocessor or a circuit in wired logic, a memory, a demodulator of the signals that may be received from terminal 1, and a modulator for transmitting information to the terminal. For example, in the case of an electronic label to which the present invention more specifically applies, this information is most often formed of a (binary) code enabling the processing system, on the side of terminal 1, to identify the product with which the label is associated.
The oscillating circuit of terminal 1 is excited by a high-frequency signal (for example, at 13.56 MHz) capable of being sensed by a transponder 10. When transponder 10 is in the field of terminal 1, a high-frequency voltage is generated across terminals 11, 12 of the transponder's resonant circuit. This voltage, after being rectified and possibly clipped, provides the supply voltage of electronic circuits 13 of the transponder.
The oscillating circuits of the terminal and of the transponder are generally tuned on the same frequency corresponding to the frequency of the excitation signal of the oscillating circuit of terminal 1. This high-frequency signal (for example, 13.56 MHz) is used as a remote supply carrier for the transponders located in the terminal's field and, when necessary, as a data transmission carrier from the terminal to this (these) transponder(s). When a transponder is in the field of a terminal 1, a high-frequency voltage is generated across terminals 11 and 12 of the transponder's resonant circuit. This voltage, after rectification and possible clipping, provides the supply voltage of electronic circuits 13 of the transponder.
The high-frequency carrier transmitted by the terminal is generally modulated in amplitude by said terminal according to different coding techniques to transmit data and/or control signals to one or several transponders in the field. In return, the data transmission from the transponder to a terminal is generally performed by modulating the load formed by resonant circuit L2, C2. The load variation (called a back modulation) is performed at the rate of a sub-carrier having a frequency (for example, 847.5 kHz) smaller than that of the carrier. This load variation can then be detected by the terminal as an amplitude variation or as a phase variation by means, for example, of a measurement of the voltage across capacitor C1 or of the current in the oscillating circuit. In FIG. 1, the measurement signal has been symbolized by a connection 5 in dotted lines connecting the junction of inductive resistor L1 and capacitor C1 to circuit 2.
The back modulation performed by the transponder may be resistive or capacitive. The present invention relates to a transponder with a resistive back modulation. The resistive back modulation includes connecting, in parallel with resonant circuit L2, C2, a resistor in series with a switch (generally, a MOS transistor). The switch is controlled according to the code to be transmitted (0 or 1) at the rate of the sub-carrier. The parallel connection is generally performed downstream of the rectifying bridge.
More and more, it is desired to integrate the entire transponder in a same silicon chip (antenna L2, capacitor C2 and circuit 13). Further, it is desired to minimize the surface area of the chip to miniaturize transponders, in particular in applications to electronic labels. An electronic label can then replace the printing of a conventional bar code to contain the identifier of a product.
A problem of conventional transponders is linked to the need to use a modulator-demodulator to convert a code (digital data, identification code of a label, etc.) to be transmitted to the read-write terminal. This need entails the necessary use of a storage (for example, a RAM, ROM, EPROM or EEPROM) of the code in binary form and of a modulation circuit upstream of the actual resistive back-modulation elements. This complexity adversely affects the miniaturization and the cost of present transponders.
Another disadvantage is that the programming of a binary code and, more generally, the writing of data in an electromagnetic transponder, requires at best a communication with a read-write terminal and a complex exchange protocol. At worst, in the case where it would be desired to store an immutable code in a one-time programming memory, it is necessary to write this code upon manufacturing in an EPROM or an EEPROM.
Another known technique is to increase the value of a resistance by optically submitting it to a laser which physically reduces its size. A disadvantage is the exorbitant price of the operation, which must be repeated for each chip. Another disadvantage is that the processing is optically visible, and thus detectable.